The present invention relates to skin moisturization, and more specifically, to a composition having a super-moisturing effect on the skin.
Physiologically, the skin is composed of an external integument called the stratum corneum, an underlying basal layer and the dermis. This entire skin organ functions as a primary moisture reservoir for the body.
The stratum corneum, which varies in thickness from approximately 15 microns on the face and the backs of the hands to 500 microns on the soles of the feet and the palms of the hands, plays a significant role in controlling the level of moisture in the skin. It is composed of keratinized cells, a natural moisturizing factor and lipids. All of these function together as a protective coating, as well as a moisture barrier to retain moisture within the skin.
In the basal area, which lies below the stratum corneum, the cells of the skin undergo change from normal cell structure to the keratinized layer of the corneum. During this change, protein breakdown products are formed, among which are pyrroledone carboxylic acids, which function as natural moisturizing compounds. Below the basal layer, lies the normal dermis of the skin, which holds and transports water to the area.
Water is extremely important to the proper physical condition and appearance of skin. Dry and chapped skin is largely the result of an insufficient level of moisture in the stratum corneum. Soft, pliable, healthy skin cannot be maintained in the absence of the proper level of moisture in the stratum corneum.
The level of moisture in the skin is dependent upon a number of factors. Among these are the water binding potential of the stratum corneum, the rate at which water is supplied to the internal layers of the stratum corneum, and the rate at which water is lost from the external surface of the skin. With these factors in mind, investigators have, for a number of years, been actively searching for ways to maintain proper levels of moisture in the skin.
Because emulsions contain two media, water and oil, thought to be important in skin moisturization, they have been widely used in formulating moisturizing compositions. Oil-in-water emulsions have received the most attention principally because until recently they were much better understood than water-in-oil emulsions. Thus, they were generally easier to prepare and gave better stability. In order to increase their moisturizing effectiveness, many workers have added humectants to these emulsions.
Desirably, a humectant for use in moisturizing cosmetic preparations should have the ability to super-moisturize the skin over extended periods of time. While the ability to prevent drying of the preparation itself may be important for aesthetic ressons as they may relate to consumer acceptance, the presence of this property in a preparation does not establish the effectiveness of the preparation as a skin moisturizer. The essential feature is whether or not the skin is moisturized. A beneficial side effect may be that the preparation itself does not dry out.
Typical of the humectants used in early work are glycerin, the polyethylene glycols, and gums. These humectants will decrease the rate of water loss from the vehicle itself and prevent crust formation in oil-in-water emulsions. However, it is reported that these compounds neither decrease the rate of water loss from, nor increase the water content of, the stratum corneum. (Shelmire Jr., J. B.; Archives Of Dermatology; 82:24-31; 1960). Shelmire asserts, in fact, that while the vehicles containing these humectant substances will themselves dry out less rapidly, they actually result in the formation of a less efficient water-retaining film than those preparations excluding them.
Thus, the early workers, believing that humectants were important, apparently overlooked an important fact--how the humectant affected the moisture level of the skin.
A later worker (Laden, U.S. Pat. No. 3,235,457) discloses certain humectants which are said to maintain a soft, smooth feeling in the skin. Among the compounds disclosed by Laden are the sodium, potassium and ammonium salts of 2-pyrrolidone-5-carboxylic acid, 1-methyl-2-pyrrolidone-5-carboxylic acid, 2-pyrrolidone-4-methyl-5-carboxylic acid, N-acetyl glycine, L-acetamido butyric acid, and N-acetyl alanine. Laden discloses that for cosmetic preparations, it is possible to employ the free carboxylic acids because the free acid will react with sodium ions naturally present on the skin or in perspiration to form the sodium salt while in place on the skin. These compounds are, however, quite costly; and, suitable alternatives and even more effective compounds would be desired.